fs: f2fs: add missing blank lines after declarations

clean up checkpatch warning:
WARNING: Missing a blank line after declarations

Signed-off-by: youngjun yoo <youngjun.willow@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>